Electric insulator having potential drop controlling means



G. PERRINS 2,576,723 ELECTRIC INSULATOR HAVING POTENTIAL DROP CONTROL G MEANS Filed May 1949 Nov. 27, 1951 I N VE NT OR fieazyeferrzw 4/3: ATTORNEY Patented Nov. 27, 1951 ELECTRIC INSULATOR HAVING POTENTIAL naor CONTROLLING MEANS George Perrins, Newcastle-under Lyme, England, assignor touBullers Limited, London, England,

a. British company Application May 2, 1949, Serial No. 90,912 In Great Britain October 11, 1946 2 Claims. (01. 174-440) This invention relates to electric insulators particularly for use in high voltage systems and of the kind provided with means for controlling the potential drop acrossthemf' For the. purpose of preventing radio interference it has'already been proposed to provide. the. areas of the insulator around the electrodes with a more or less conducting coating leaving an area of nonconducting surface in between them.

It has also been proposed to control the potential drop across the surface of an insulator by covering the entire surface of the insulator with a semi-conducting layer so as toprovide a definite overall ohmic resistance.

It has also been proposed to provide the insulator surface with one ormore bands of conducting coating extending between the electrodes.

It is not generally found possible or convenient to employ a conducting coating which has a zero temperature coefficient of resistivity but it will be appreciated that the resistance characteristics of an insulator should remain constant irrespective of variations in temperature.

According to this invention the surface of an insulator is provided with two conducting coatings one having a positive temperature coefficient of resistance and the other a negative temperature coefficient of resistance which coatings are arranged to provide between them a resistance the value of which is independent of changes in temperature.

The invention is particularly applicable to an insulator in which the coatings are arranged in the form of bands extending between the metal electrodes, in which case one or more bands may comprise throughout their length a coating of positive temperature coefficient of resistance while one or more other bands may have a negative temperature coefiicient of resistance throughout their length.

Alternatively, a part or parts of a length of band may comprise a coating or positive temperature coefficient of resistance while another part or parts may have a negative coefficient of resistance.

Thus, in the form of invention for use with a cap and pin insulator a band having a positive temperature coefiicient of resistance may extend radially outwards from the metal cap of the insulator to the periphery of the shed where it meets a band having a negative temperature coefiicient of resistance which extends radially inwards to the cavity for the pin.

Alternatively, on one side of the insulator a.

band of positive temperature coefficient of resistanceextends from said cap around the edge and back to the other electrode while on the other side of a band of negative temperature coeflicient of resistance extends in a similar manner.

In any of the arrangements referred to above ceramic glazes or other materials having the required resistivities and temperature coefficients of resistance may be employed.

The following is a description of two embodiments of the invention, reference being made to the accompanying drawings, in which:

Figure I is a section through an insulator having a conducting band of positive temperature coefficient of resistance on top of the shed and negative coeflicient of resistance on the bottom of the shed;

Figure 2 is a plan of the arrangement shown in Figure 1;

Figure 3 is a similar view to Figure 1 showing an insulator provided with two radial bands of conducting material one having a positive temperature coefficient of resistance and the other a negative one, and

Figure 4 is a plan of the arrangement shown in Figure 3.

Referring to the arrangement shown in Figures 1 and 2, the insulator is of conventional cap and pin type comprising a head In surrounded by a metal cap ll having a socket 12 therein for engagement by a pin of a neighbouring insulator. A cavity l3 extends up into the head and accommodates a pin M. The head is formed integral with an encircling shed l5 provided with circumferential ribs IS. A band I1 of conducting material having a positive temperature coefiicient of resistance, such for example as is formed by firing a ceramic material comprising ZnO and/or NiO and FeOs, as set out in British Patent No. 577,748, extends from the base of the head l0 radially outwards to the periphery of the shed where it meets another band [8 of conducting material of. a negative temperature coefiicient of resistance such for example, as finely divided metallic nickel which extends radially inwards over the ribs l6 and terminates in the mouth of the cavity I3.

In the arrangement shown in Figures 3 and 4, two bands l9 and 20 of conducting material are provided which extend radially on opposite sides of the insulator, one of which bands l9 may be formed from a material which has a positive temperature coefiicient of resistance and extends from the base of the head around the periphery of the shed and then radially inwards over the ribs IE to the mouth of the cavity [3, while the other band 20 may be formed from a conducting material having a negative temperature coefficient of resistance and extends in a similar manner from the base of the head around the shed to the mouth of the cavity. By these means, by suitably selecting the resistivities of the bands, the resistance of the insulator surface may be maintained at a required value irrespective of the temperature conditions. The conducting bands may be provided by ceramic glazes or other materials having the required resistivities and temperature coefficient of resistance.

In the arrangement shown in Figure 1, more than one compound band may be provided, and similarly in Figure 2 more than one positive band and more than one negative band may also be provided. Furthermore, the extremities of the bands may be arranged to extend over the whole of the outer surface of the head portion of the insulator and over the whole surface of the cavity of the insulator as described in the specification of British application No. 30,364/46.

I claim:

1. A high voltage electric insulator arranged for connection between two metal electrodes for controlling the potential drop across the electrodes, the surface of which insulator is provided with two high resistance coating bands arranged electrically to connect the two electrodes and each being over substantially the whole of its area in contact with the surface of the insulator and out of contact with the other hand, one of which bands has a positive temperature coefficient of 3 resistance and the other a negative temperature 25 file of this patent:

coefiicient of resistance, which two bands are arranged to provide between them a resistance the value of which is independent of changes in temperature.

2. A high voltage electric insulator arranged for connection between two metal electrodes for controlling the potential drop across the electrodes, the surface of which insulator is provided with two high resistance coating bands arranged end tovend between the two electrodes, so that each contacts over substantially the whole of its area with the surface of the insulator, and is out of contact over substantially the whole of its area with the other band, one of which coatings has a positive temperature coefiicient of resistance, the other a negative temperature coeflicient of resistance, which band resistances are arranged to provide between them a resistance the value of which is independent of the changes of temperature.

GEORGE PERRINS.

REFERENCES CITED The following references are of record in the UNITED STATES PATENTS Great Britain Feb. 11, 1949 

